package ejava.jpa.example.validation;
   
   import java.util.Calendar;
   import java.util.Date;
   import java.util.GregorianCalendar;
   
   import javax.validation.ConstraintValidator;
   import javax.validation.ConstraintValidatorContext;
   
  public class MinAgeValidator implements ConstraintValidator<MinAge, Date>{
  	int minAge;
  
  	@Override
  	public void initialize(MinAge constraint) {
  		this.minAge = constraint.age();
  	}
  
  	@Override
  	public boolean isValid(Date date, ConstraintValidatorContext ctx) {
  		if (date==null) { return true; }
  		//get today's date
  		Calendar latestBirthDate = new GregorianCalendar();
  		//determine min date that meets age requirement
  		latestBirthDate.add(Calendar.YEAR, -1*minAge);
  		
  		//get calendate date of object
  		Calendar birthDate = new GregorianCalendar();
  		birthDate.setTime(date);
  
  		//return whether meets criteria
  		if (birthDate.after(latestBirthDate)) {
  			String errorMsg = String.format("%d is younger than minimum %d", 
  					getAge(birthDate), 
  					minAge);
  			ctx.buildConstraintViolationWithTemplate(errorMsg)
  				.addConstraintViolation();
  			return false;
  		} else {
  			return true;
  		}
  	}
  	
  	private int getAge(Calendar birth) {
  		Calendar now = new GregorianCalendar();
  		int years = now.get(Calendar.YEAR) - birth.get(Calendar.YEAR);
  		int months = now.get(Calendar.MONTH) - birth.get(Calendar.MONTH);
  		if (months < 0) { years -= 1; }
  		else if (months==0) {
  			int days = now.get(Calendar.DAY_OF_YEAR) - birth.get(Calendar.DAY_OF_YEAR);
  			if (days < 0) { years -= 1; }
  		}
  		return years;
  	}
  }